It has been revealed in this laboratory that severe combined immunodeficient (SCID) mice, in spite of their inability to generate a specific immune response to infection with attenuated and virulent M. tuberculosis (Mtb), can nevertheless exert an appreciable restrictive influence on the growth of Mtb in their lungs and other organs. This growth restrictive mechanism can be abolished by treating Mtb-infected SCID mice with hydrocortisone, or with monoclonal antibodies (mAbs) directed against TNFalpha, IFNgamma, or neutrophils. The goal of the proposed research is to gain a general understanding of the cellular basis of this non-immune growth restrictive mechanism in the lungs of SCID mice. It is hypothesized that Mtb growth restriction is expressed by macrophages, the antimycobacterial function of which is upregulated as part of a non-immune cellular response to infection. This response will be analyzed histologically to determine whether SCID mice infected by aerosol Mtb is confined to the cytoplasm of lung macrophages while growth restriction is being expressed and after it is abolished by treating mice with the above mentioned reagents. In the latter case, increased bacillary load per macrophage and dissociation of granulomas is expected. The possibility that expression of Mtb growth restriction depends on an influx of monocytes, NK cells and neutrophils from blood will be determined. It will also be determined whether Mtb growth restriction is associated with TNFalpha and IFNgamma production in the lung. A spot ELISA assay will be used to enumerate and identify the cells that produce these cytokines. The need for IFNgamma and TNF to upregulate the Mtb growth restrictive mechanisms of macrophages, and the ability of hydrocortisone to abolish the mechanism will be studied in vitro. It will be determined whether Mtb growth restriction in the lung serves to retard the hematogenous dissemination of infection to other organs.